1. FIELD OF THE INVENTION
The present invention pertains generally to infrared thermometers primarily for agricultural purposes and more specifically, to an improved infrared thermometer apparatus including means for making more accurate temperature measurements by compensating for reflected sky radiation.
2. PRIOR ART
It is well-known in the art to utilize infrared thermometers. Such temperature measuring devices are used in measuring the temperature of an object or a series of closely spaced adjacent objects by measuring the relative infrared radiation levles using an infrared detector such as a thermopile. In these devices it is typical for a circuit to be used to convert the signal produced by the infrared detector and associated circuitry such as, for example, an amplifier and a linearization circuit into an output signal capable of indicating a numerical value. This output signal is then utilized in a meter type structure such as a digital display or a known meter movement in order to indicate a numerical temperature value. Exemplary of such prior art devices are those disclosed in U.S. Pat. Nos. 4,078,179 and 4,132,902, both issued to Everest, one of the joint inventors of the present invention. Still another exemplary illustration of an infrared temperature measurement device is disclosed in the Everest U.S. patent application Ser. No. 069,269 filed Aug. 24, 1979, now U.S. Pat. No. 4,301,682.
The temperature indication of such devices normally corresponds to the temperature of an object or a series of objects as indicated by the relative levels of radiation of such objects and of the infrared detector itself. Such infrared thermometers are considered to be very desirable for many purposes but they are particularly suitable for agricultural applications where they are normally constructed as hand-held gun-like instruments which can be easily and conveniently used.
Unfortunately, such infrared thermometers when used for agricultural applications are inherently inaccurate because they do not account for reflections from the plant surfaces produced by sky emanating radiation. This failure to take into account plant reflectance of sky radiation can adversely affect the accuracy of the temperature reading to varying degrees depending upon sky conditions and the reflection coefficient of the vegetation to which the temperature measuring instrument is directed. The inaccuracy resulting from the failure to account for sky radiation reflection may, in fact, become fairly significant because of the inherent small difference between the infrared temperature of the plants and the ambient air temperature, both of these parameters being used in at least one prior art device for making stress degree measurements which require a comparison of the temperature of a plant or of a crop canopy with the ambient temperature adjacent the plant or crop canopy to determine whether or not a plant or crop is stressed due to a need for watering or irrigation. Such a stress degree measuring instrument is disclosed in applicant's prior U.S. patent application Ser. No. 069,269, now U.S. Pat. No. 4,301,682, which is incorporated herein by reference.